1. Field of the Invention
The present invention relates to an electrophotographic photoconductor, and more particularly to an electrophotographic photoconductor comprising a photoconductive layer which contains a compound comprising a charge generating moiety and a charge transporting moiety in the molecule thereof, prepared by chemically bonding a molecule of a charge generating material and a molecule of a charge transporting material.
In addition, the present invention also relates to bisazo compounds and trisazo compounds which serve as the above-mentioned compounds with a charge generating moiety and a charge transporting moiety in the molecule thereof and work as the organic photoconductive materials for use in the electrophotographic photoconductor, and intermediates for producing the above-mentioned disazo and trisazo compounds.
2. Discussion of Background
There are conventionally known an inorganic electrophotographic photoconductor comprising selenium or alloys thereof, and an inorganic photoconductor in which zinc oxide sensitized by a dye is dispersed in a binder resin. As the organic electrophotographic photoconductor, on the other hand, there is well known a photoconductor comprising a charge transporting complex of 2,4,7-trinitro-9-fluorenone (hereinafter referred to as TNF) and poly-N-vinylcarbazole (hereinafter referred to as PVK).
However, while these photoconductors have many advantages, they have several shortcomings. For instance, a selenium photoconductor, which is widely used at present, has the shortcomings that the manufacturing conditions are difficult and, accordingly its production cost is high. In addition, it is difficult to work it into the form of a belt due to its poor flexibility, and it is so vulnerable to heat and mechanical shocks that it must be handled with the utmost care.
The production cost of a zinc oxide photoconductor can be decreased because the zinc oxide photoconductor can be obtained by merely applying a coating liquid containing cheap zinc oxide particles to a support. However, not only the photosensitivity of the zinc oxide photoconductor is low, but also the mechanical properties, such as surface smoothness, hardness, tensile strength and wear resistance are poor. Accordingly, when such a photoconductor is repeatedly used in a copying machine for plain paper, there are many problems in its durability.
The photosensitivity of the photoconductor comprising the previoiusly mentioned TNF and PVK is low, so that it is difficult to employ this kind of photoconductor in the high speed copying machine.
To eliminate such shortcomings of the above-mentioned photoconductors, studies have been extensively conducted, and in particular, a variety of organic photoconductors have been proposed. Especially, some attentions have been paid to a laminated photoconductor as a photoconductor for use in the copying machine for plain paper because the photosensitivity of this type of photoconductor is higher and the chargeability is more stable than those of the conventional organic photoconductors. The aforementioned laminated photoconductor is prepared by providing a thin layer (i.e. a charge generation layer) comprising an organic dye on an electroconductive support, and then a layer (i.e. a charge transport layer) mainly comprising a charge transporting material on the charge generation layer. Some of the laminated organic photoconductors have been put to practical use.
To be more specific, the following laminated photoconductors are well known:
(1) A laminated photoconductor as disclosed in U.S. Pat. No. 3,871,882, comprising a charge generation layer of a thin-layered type prepared by vacuum-deposition of a perylene derivative, and a charge transport layer comprising an oxadiazole derivative. PA1 (2) A laminated photoconductor as disclosed in Japanese Patent Publication 55-42380, comprising a charge generation layer of a thin-layered type prepared by coating of an organic amine solution containing chlorodiane blue, and a charge transport layer comprising a hydrazone compound.
However, those conventional laminated photoconductors have their own drawbacks although they have many advantages.
For instance, the photosensitivity of the above-mentioned laminated photoconductor (1) comprising the perylene derivative and oxadiazole derivative is too low to be used in the high speed copying machine although the photoconductor (1) is applicable to the copying machine for general use. In addition, the perylene derivative, that is a charge generating material to control the spectral sensitivity of the photoconductor, has no absorption in the whole visible region, so that this kind of photoconductor cannot be used in a color copying machine.
The laminated photoconductor (2) comprising the chlorodiane blue and hydrazone compound has a relatively high photosensitivity, but it has the problems in the production conditions because an organic amine such as ethylenediamine which must be handled with great care is generally used as a solvent for the preparation of a coating liquid for the charge generation layer.
In general, the organic photoconductor comprises the charge generating material and the charge transporting material, as previously mentioned. As stated in "IS&T's 10th International Congress on Non-Impact Printing Technologies 1994, page 239", the sensitizing effect of the charge generating material by the charge transporting material is known as a factor in determination of the high sensitivity of the organic photoconductor. In addition, according to the above-mentioned reference, a site for generating a charge carrier when exposed to light, namely a photo-carrier generation site or a charge carrier injection site is located on the interface between a charge generating molecule and a charge transporting molecule. However, the charge generating material for general use is only slightly soluble in most organic solvents, so that the charge generating material is dispersed in the form of particles in the charge generation layer. Therefore, the number of photo-carrier generation sites or charge carrier injection sites is limited because the charge generating material exists in the form of finely-divided particles although the charge transporting material is in the form of a molecule, thereby restraining the increase of sensitivity of the photoconductor. Conversely speaking, it is considered that the sensitivity of the photoconductor can be improved by increasing the number of sites where the charge generating molecule and the charge transporting molecule come in contact with each other, anyway.
It is conventionally known that various azo compounds are effective as charge generating materials in the previously mentioned laminated electrophotographic photoconductor. The laminated photoconductor is constructed in such a manner that (i) a charge generation layer comprising a charge generating material capable of generating charge carriers when exposed to light, and (ii) a charge transport layer comprising a charge transporting material capable of efficiently injecting the above-mentioned charge carriers in the charge transport layer and transporting the same, are successively overlaid on an electroconductive support. To prepare the charge generation layer, the charge generating material may be vacuum-deposited on the electroconductive support. Alternatively, a solution containing the charge generating material or a dispersion prepared by dispersing the finely-divided particles of the charge generating material in a resin solution may be coated on the electroconductive support. On the other hand, the charge transport layer generally comprises the charge transporting material and a binder resin.
As the azo compounds for use in the above-mentioned photoconductor, there are conventionally known benzidine bisazo compounds as disclosed in Japanese Laid-Open Patent Applications 47-37543 and 52-55643; and stilbene bisazo compounds as disclosed in Japanese Laid-Open Patent Application 52-8832.
However, the photosensitivity of the laminated electrophotographic photoconductors employing the aforementioned conventional azo compounds is generally low, so that such photoconductors are not suitable for the high-speed copying machine.